Head over to the Eastern side of the plantation. Go to the Southeastern edge of the search zone and look over the wall. Mark the nearby Soldiers with Eagle Vision. When they are looking away, climb onto the wall and jump into the Sugar cane. Lure in the nearby guard and take him down. Deal with the other guards similarly or just use the Dart Gun to knock them out. Head over to the bell after that and disable it.
Head to the West, climbing up into the trees and using them to cross over to the Western side of the plantation. From the tree branches above, you can use the Dart gun to knock out some of the guards below. Leave the Brute up as you can take him out last and get credit toward your other optional objective. Do note that tagging him with a Berserk Dart will also give you the kill credit.